Many different types of spectroscopic instruments have been used for many different purposes. For example, one can analyze the wavelength of light passed through the instrument and determine the characteristics about the light source.
One such type of spectroscopic instrument is commonly known as the Czerny-Turner type spectrograph. Generally speaking, this instrument comprises an entrance aperture for receiving a source of light, collimating optics, a dispersive element, such as a planar diffraction grating, and focusing optics for creating a spectrally decomposed image of the input. Though the dispersive element, collimating and focusing optics might be of the refractive type (such as prisms or lenses), the most practical type is the reflective type (such as reflective-type gratings and mirrors). The reflective type is preferred because it can be used over a wider spectral range when compared to the refractive type. For example, the refractive type is generally unworkable for electromagnetic radiation in the infrared and ultraviolet wavelength ranges. Problems associated with refractive type optics are discussed by R. Yeo in "Imaging Spectroscopy", published in Spectroscopy Europe magazine, p. 20 (Apr. 6, 1992).
Due to the nature of the reflective-type optical elements involved, it is necessary that the collimating and focusing optics be arranged in an "off-axis" configuration. This means that it is not possible to position the elements such that the normals to all the optical elements lie along a single straight line.
Unfortunately, arrangement in the off-axis configuration can introduce relatively severe aberrations into the system. The most important of these aberrations are coma and astigmatism. Coma is the "blurring" of the image along the spectral (or horizontal) range, while astigmatism is elongation of the image along the spatial (or vertical) range.
As known in the art, coma can be sufficiently corrected, for most purposes, for example, by arranging two off-axis mirrors to adjust the off-axis angles so that they have opposite orientations and magnitudes; this results in one coma being subtracted from the other, thus "cancelling" the coma. Arrangements that allow one to compensate for coma are discussed in U.S. Pat. No. 3,011,391 to W. G. Fastie; U.S. Pat. No. 3,414,355 to W. G. Fastie et al.; and in an article entitled "Theory and Principles of Monochromators, Spectrometers and Spectrographs" by M. V. R. K. Murty, published in volume 13 of Optical Engineering, pp. 23-39 (1974).
Several prior art patents discuss methods and/or apparatuses for correcting an astigmatism, the other aberration introduced into off-axis systems. For example, U.S. Pat. No. 4,932,768 to Gobeli describes using two toroidal mirrors--one as the focusing mirror and one as the collimating mirror. U.S. Pat. No. 5,192,981 to Slutter et al. attempts to reduce astigmatism by use of a single toroidal collimating mirror in combination with a spherical focusing mirror.
Unfortunately toroidal mirrors are extremely expensive to manufacture with the quality necessary for spectrographic applications. Furthermore, toroidal mirrors can only be economically manufactured with relatively high tolerances (e.g. wavelength/2) as compared with conventional mirrors (e.g. wavelength/10) and relatively degraded performance is thus experienced.
Another attempt to reduce astigmatism is discussed in U.S. Pat. No. 5,384,656 to Schwenker. Schwenker discloses replacing a planar diffraction grating with a grating ruled on a cylindrical blank. Although cylindrical surfaces are much less expensive to manufacture than toroidal surfaces and thus provide a more economical way to correct astigmatism, these gratings on a cylindrical surface are more difficult to manufacture and, hence, far more expensive than conventional planar gratings, and they are also difficult to locate in the marketplace.
It is also known that a cylindrical lens positioned between the input aperture and a collimating mirror can correct astigmatism. See "Imaging Spectroscopy", supra, at page 20. However, as mentioned before, use of refractive optics (such as lenses) create chromatic aberrations and are, for practical purposes, unusable for a wide spectral range of instruments.
It is also known that placement of a reflective-type cylindrical mirror, instead of a refractive-type lens mentioned above, positioned between an input aperture and a collimating mirror can reduce astigmatism, as described in "Optics of Spectral Instruments", by I. Peisakhson, Leningrad, USSR (1975). This approach, however, is not applicable to compact (short focal length), low F-number (high throughput) instruments. First, it is generally not convenient to construct the instrument with an additional element in the small distance (i.e., 100-150 millimeters) between the entrance aperture and the collimating mirror. This is especially true if other necessary accessories, such as a shutter, must also be positioned in this same space. Second, at low F-numbers, the off axis angle of the cylindrical mirror as taught by the prior art must be rather large (ten degrees or higher). This, when combined with the fact that the mirror as taught by the prior art is positioned in a highly divergent beam, causes significant deterioration in the overall performance of the instrument.
While correction of astigmatism is highly desirable in Czerny-Turner type spectrographs and monochromators, it becomes critical when multielement detectors are used. Multielement detectors are made up of a plurality of pixels; generally each pixel size in the spectral direction is about 20 micrometers. Typically, multielement detectors are two dimensional, that is, they have multiple elements along both the spectral and spatial direction. As a result, multitrack spectroscopy is possible; that is, multiple parallel spectral tracks from different object points along the input slit can be registered and analyzed simultaneously.
With multitrack spectroscopy, it is critical to have a sharp image not only in the spectral direction but also in the spatial direction. Otherwise, one track can "yblur" into the other. For example, a blurred image on a first track is generally not useful to a researcher, and can be problematic when the image on the first track "blurs" into an image on a neighboring second track. This "blurring" can occur when the image is not corrected for astigmatism. Thus, it is extremely important that images be corrected for astigmatism when multielement detectors are used.
Another problem associated with spectrographs when multielement detectors are used is sometimes referred to as "reentrant radiation". Reentrant radiation is that electromagnetic radiation that reflects from the detector surface and reenters the instrument, and is reflected back sometimes several times for re-detection by the detector. Reentrant radiation that is re-diffracted and then re-detected by a detector is referred to herein as reentrant spectra. Because multielement detectors are typically made of semiconductor material, such as silicon, they are typically highly reflective, thus generating much reentrant radiation, which may undesirably be re-detected as reentrant spectra. U.S. Pat. No. 4,932,768 to Gobeli discloses tilting the image plane eleven (11) degrees relative to a plane orthogonal to the central ray from the focusing mirror. This results in all radiation reflected from the detector completely missing the focusing mirror and eliminating reentrant spectra. Unfortunately,.this angle is very large and may interfere with the performance of the instrument.
What is desired, therefore, is a spectroscopic instrument which has an astigmatism-reduced image, which is more economical than conventional spectroscopic instruments used to correct astigmatism, which does not utilize any difficult to manufacture (and costly) toroidal mirrors or non-planar gratings, which can correct astigmatism such that the resulting image is useable with multielement detectors, and which is designed so as to re-direct reentrant radiation so that it is not re-diffracted by a diffractor.